Web Animations CSS: Fix assertion in AnimatableLength::toCSSPrimitiveValue
Web Animations CSS: Fix assertion in AnimatableLength::toCSSPrimitiveValue

This patch changes the assertion in AnimatableLength::toCSSPrimitiveValue
to not check the unit type if the value is a calc, since the unit type
value is meaningless when we're a calc (we should probably just make calc
a unit type...)

BUG=312409

Review URL: https://codereview.chromium.org/54083002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@161029 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs